Summary. The yeast cells Saccharomyces cerevisiae grown up tostationary phase under either anaerobic conditions, or aerobic conditions in the presence of a respiratory inhibitor, antimycin A, had distinctive giant mitochondrial nucleoids (mt-nucleoids) (apparent diameter 0.6-0.9 ~tm) in contrast with the small mt-nucleoids (apparent diameter 0.2-0.4 ~m) in respiratory-sufficient cells grown aerobically, as revealed by DAPI-fluorescence microscopy. The cytoplasmic respiratory-deficient cells (rho-cells), which were induced by treatment of wild-type cells with ethidium bromide, showed both giant and small mt-nucleoids of irregular size. In order to examine the structural and functional differences between giant and small mtnucleoids, the former were successfully isolated from spheroplasts of three different cells by differential centrifugation and centrifugation on a discontinuous sucrose gradient. The isolated giant mt-nucleoids were intact in the morphology and were free of significant contamination by nuclear chromatin. The number of protein components involved in each of three different giant mt-nucleoids was similar to the number in small mt-nucleoids from aerobically grown cells, though a few noticeable differences were also recognized. DNAbinding proteins with molecular masses of 67 kDa, 52 kDa, 50 kDa, 38 kDa, 26 kDa, and 20 kDa were the main components of small mtnucleoids from aerobically grown cells as detected by chromatography on native DNA-cellulose. In contrast, the 67 kDa and 52 kDa proteins were hardly detected in corresponding fractions of giant mtnucleoids from anaerobically grown cells and from rho-cells grown aerobically. On the other hand, mt-nucleoids from aerobically grown cells in the presence of antimycin A seemed to lack the 67 kDa protein but to have a small amount of the 52 kDa protein. This is the first demonstration of the variance of protein species involved in yeast mt-nucleoids according to the respiratory activity of mitochondria.

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Mitochondria and mitochondrial nucleoids (mt-nucleoids) of the yeast, Saccharomyces cerevisiae were vitally double-stained with two fluorochromes, 3, 3'-dihexyloxacarbocyanine iodide [ Di0C6(3) ] and 4', 6-diamidino-2-phenylindole (DAPI), respectively in different types of cells during the life cycle. Budded cells at logarithmic phase had a single, branched, giant mitochondrion, a part of which migrated into the daughter cell prior to nuclear division. Mt-nucleoids appeared as a string-of-beads within a giant mitochondrion. Single cells at stationary phase had numerous fragmented mitochondria with dot-like mt-nucleoids. When stationaryphase cells were transferred to the sporulation medium, fragmented mitochondria fused with each other and a network of mitochondrion changed its configuration as the meiotic division proceeded. Finally, mitochondria formed four rings which enclosed each daughter nucleus in premature spores. Mitochondria were hardly stained with Di0C6 (3) in mature spores. Fused mitochondria were clearly observed again in cells at germination and outgrowth. When cells of opposite mating type were mixed in mating medium, zygote formation and first bud emergence occurred in 4 h. Just after mating, string-like and fragmented mitochondria transmitted from each parent cell were separately visible on both sides of a dumbbell-shaped zygote. In contrast, extensive fusion of mitochondria occurred in zygotes with a first bud.

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SummaryThe DNA content of individual mitochondrial nucleoids (mt-nucleoids) in the stationary-phase cells of the yeast Saccharomyces cerevisiae was quantitatively measured by a combination of DAPI-staining and use of a video-intensified microscope photon counting system (VIMPCS). The mt-nucleoids in the cells which were grown aerobically in AN medium contained, on average, 1.5 mitochondrial DNA (mtDNA) molecules per mt-nucleoid, and the mt-nucleoids that were isolated from these cells contained 1.1 mtDNA molecules per mt-nucleoid. In contrast, the giant mt-nucleoids that appeared in the cells grown anaerobically in the same medium contained, on average, 20.3 mtDNA molecules per mt-nucleoid, and the giant mt-nucleoids that were isolated from these cells contained, on average, 13.1 mtDNA molecules per mt-nucleoid. These results, together with the previous data, demonstrated that the DNA content of individual mt-nucleoids and the number of mtnucleoids in a cell were distinctly changed by the culture conditions of the yeast cells, whereas the total mtDNA content per cell did not vary significantly when the culture conditions were changed.